The following abbreviations may be found herein:
3GPPthird generation partnership projectACKacknowledgementCRCcyclic redundancy checkDAIdownlink assignment indexDCIdownlink control informationDLdownlinkeIMTA systemssystems with TDD interference management andtraffic adaptationeNBnode B/eNodeB/base stationFDDfrequency division duplexHARQhybrid automatic repeat requestHARQ-ACKhybrid automatic repeat request acknowledgmentLTElong term evolutionLTE-Along term evolution advancedNACKnegative acknowledgement(E)PDCCH(enhanced) physical downlink control channelPDSCHphysical downlink shared channelPUCCHphysical uplink control channelPUSCHphysical uplink shared channelRel.Release (e.g. LTE Rel. 11 means LTE Release 11)SIBsystem information block (e.g. SIB-1 or SIB1 meanssystem information block type 1)SIFsubframe indicator fieldSPSsemi-persistent schedulingTDDtime division duplexTPCtransmit power controlUCIuplink control informationUEuser equipmentULuplink
LTE wireless communication systems aim to provide enhanced services including higher data rates and lower latency with reduced cost. One benefit of deploying LTE TDD systems is to enable configurable asymmetric UL-DL resource allocations in a radio frame. Typically if more data is to be sent in DL, there can be a higher number of DL subframes configured in a radio frame to accommodate the greater DL data volume. In previously-proposed LTE TDD systems, asymmetric resource allocation is achieved by providing seven different semi-statically configured UL-DL subframe configurations for a given radio frame, as specified in Table 4.2-2 of 3GPP TS 36.211 v 10.5.0 (2012-06) which is extracted below.
TABLE 4.2-2Uplink-downlink configurationsUplink-Downlink-downlinkto-Uplinkconfigu-Switch-pointSubframe numberrationperiodicity012345678905msDSUUUDSUUU15msDSUUDDSUUD25msDSUDDDSUDD310msDSUUUDDDDD410msDSUUDDDDDD510msDSUDDDDDDD65msDSUUUDSUUD
It can be seen that the different TDD UL-DL configurations in the table above provide between 40% and 90% DL subframes, and in conventional practice the UL-DL configuration in use at an eNB is informed to the UE (and changed) only via system information on the broadcast channel. The UL-DL configuration is only configured semi-statically and so may not adapt/match to the instantaneous traffic situation. This is inefficient in terms of resource utilization, particularly in small cells or cells with a small number of users where the traffic situation can often change more frequently or rapidly.
To address this inefficiency, a flexible TDD configuration study item for LTE-A Release 11 was completed. Evaluations in the study item revealed possibly significant performance benefits by allowing TDD UL-DL reconfiguration based on traffic adaptation in small cells. The studies also recommended interference mitigation schemes for systems with TDD UL-DL reconfiguration.
For asymmetric UL-DL configuration and flexible TDD allocation, there are several challenges to overcome before any implementation may be considered viable. One challenge relates to reference HARQ-timing used for UL and DL when TDD reconfiguration may happen as frequently as on a radio frame basis. It is thought that addressing certain difficulties associated with HARQ-ACK feedback for DL transmission (particularly difficulties related to the timing coupling between UL grant and DL scheduling in flexible-TDD systems) may help in this regard.
Diagram (100) in FIG. 1 illustrates the HARQ timing rule specified in LTE Rel. 8, 9, 10 and 11. As specified in LTE Rel. 8-11 and illustrated in diagram (100), one UL subframe in radio frame (n) or (n+1) is responsible for carrying HARQ-ACK feedback for M DL and/or special subframes in radio frame (n), where M is the size of a DL association set K as specified in Table 10.1.3.1-1 of 3GPP TS 36.213 (which is the lower table (120) in FIG. 1). In table (120), the DL association set K is defined for each UL subframe for the different TDD UL-DL configurations. For instance, in TDD UL-DL configuration #3 (125), UL subframe #2 (124) is responsible for carrying HARQ-ACK feedback for DL transmissions which happened k subframes earlier, where the value(s) of k is/are specified in table (120). In this instance (i.e. for UL subframe #2 in TDD UL-DL configuration #3) the values of k are 7 (123), 6 (122) and 11 (121). Hence, UL subframe #2 (124) in TDD UL-DL configuration #3 (125) is responsible for carrying HARQ-ACK feedback for the 7th, 6th and 11th earlier subframes. As a result, in TDD UL-DL configuration #3 (125), UL subframe #2 (114) in radio frame n+1 is responsible for carrying HARQ-ACK feedback for the following subframes in frame n: special subframe #1 (111) (for k=11), DL subframe #5 (112) (for k=7) and DL subframe #6 (113) (for k=6). Thus, in this example, the predefined DL association set K is {7, 6, 11} or {DL subframe #5, DL subframe #6, special subframe #1} and M=3.
In the present context, the subframe number of a particular subframe in a radio frame may also be referred to as the subframe index. For example, for UL subframe #2 in a given radio frame, the subframe index is 2. Likewise, for UL subframe #5 in a given radio frame, the subframe index is 5.
In cases where only one serving cell is configured for a UE, a 2-bit Downlink Assignment Index (DAI) field, VDAIUL, in DCI format 0/4 represents the total number of subframes with PDSCH transmissions and with PDCCH/EPDCCH indicating downlink SPS release sent to the corresponding UE within all the subframes of the DL association set. The value of VDAIUL includes all PDSCH transmissions, with and without corresponding PDCCH/EPDCCH, within all subframes in the DL association set K.
In timing diagram (200) in FIG. 2A, TDD UL-DL configuration #1 is broadcast by a base station using SIB-1. According to the LTE Rel. 8-11 HARQ timing rule discussed above, the HARQ-ACK feedback for DL subframe #0 (201) and/or for special subframe #1 (202) is reported on UL subframe #7 (203). Hence, the relevant DL association set K related to UL subframe #7 (203) is {7,6} or {DL subframe #0 (201), special subframe #1 (202)}. The total number of subframes with PDSCH transmissions and with PDCCH/EPDCCH indicating downlink SPS release within this DL association set {DL subframe #0, special subframe #1} related to UL subframe #7 (203) is indicated by the value of VDAIUL in UL grant (204). UL grant (204) is sent by the base station on special subframe #1 for PUSCH transmission on UL subframe #7 according to the UL grant timing specified in table 8-2 of TS36.213 (see immediately below). Notably, the UL grant (204) is sent on special subframe #1 (202) which is the latest subframe in the DL association set related to UL subframe #7 (203).
TDD UL/DLConfigu-DL subframe number nration01234567890464616464244344444454677775
As another example, according to the HARQ timing rule discussed above, in TDD UL-DL configuration #1 the HARQ-ACK for DL subframe #5 and/or for special subframe #6 in frame n is reported on UL subframe #2 in frame n+1. The total number of subframes with PDSCH transmissions and with PDCCH/EPDCCH indicating downlink SPS release within the DL association set {DL subframe #5 in frame n, special subframe #6 in frame n}(or {7,6}) related to UL subframe #2 in frame n+1 is indicated by VDAIUL sent in the UL grant (206). UL grant (206) is sent by the base station on special subframe #6 in frame n for PUSCH transmission on UL subframe #2 in frame n+1 (this is again according to UL grant timing specified in table 8-2 above). And again, the UL grant (206) is sent on special subframe #6 in frame n which is the latest subframe in the DL association set related to UL subframe #2 in frame n+1.
Thus, it can be observed that all UL grants (204, 205, 206 and 207 in FIG. 2A) are transmitted in the DL/special subframe which is the latest DL transmission instance in the related DL association set. Therefore, the UL grant timing allows the base station to count all of the PDSCH transmissions and PDCCH/EPDCCH transmissions indicating SPS release which happened within a DL association set and to notify the intended UE of the number of said transmissions using VDAIUL in the UL grant. In other words, because UL grants are sent on the last subframe in the related DL association set, the base station such as eNB can count the number of subframes in that DL association set (up to and including the said last subframe on which the UL grant is sent) which included PDSCH transmissions and/or PDCCH/EPDCCH transmissions indicating SPS release, and it can communicate this number to the intended UE via VDAIUL sent in the UL grant. Then, by interpreting VDAIUL in the received UL grant, the UE can tell the total number of PDSCH transmissions and PDCCH/EPDCCH transmissions indicating SPS release that it should have received. This then enables the UE to know whether to send ACK or NACK in the relevant UL subframe, depending on whether or not all PDSCH transmissions and PDCCH/EPDCCH transmissions indicating SPS release are successfully received.
The UL grant timing diagram (200A) in FIG. 2B shows the timing of UL granting and DL HARQ-ACK timing for all seven TDD UL-DL configurations. It can be seen that an UL grant is always transmitted no earlier than the last DL transmission instance within the DL association set related to the particular UL subframe. There is therefore a coupling between the timing of UL grant and DL transmission scheduling. As a result, it could be problematic if the timing of UL grant and DL transmission scheduling were to follow different reference timing for a particular TDD UL-DL configuration.
In spite of this, in flexible-TDD systems, reference timing configurations are expected to be selected independently for DL and UL respectively. In other words, in flexible-TDD systems, the timing of UL grant and DL transmission scheduling are expected to follow different reference timing for a particular TDD UL-DL configuration. The abovementioned coupling between the timing of UL grant and DL transmission scheduling may therefore have the potential to cause problems, including in relation to TDD HARQ-ACK bundling and TDD HARQ-ACK multiplexing operation, and it may be desirable if such problems (or one or some of them) could be ameliorated or at least reduced. Ameliorating or reducing such problems (or one or some of them) may help to achieve gains from flexible-TDD systems.
It is to be clearly understood that mere reference herein to previous or existing apparatus, systems, methods, practices, publications or other information, or to any associated problems or issues, does not constitute an acknowledgement or admission that any of those things individually or in any combination formed part of the common general knowledge of those skilled in the field, or that they are admissible prior art.